Cast polymeric tools are well known for making tools to stamp sheet metal or mold plastics. However, all cast polymeric materials such as nylons, polyesters, epoxies and urethanes shrink and/or change shape as they harden and cure. Therefore, it has not been possible to simply cast large tools to size. The problem is further aggravated by the fact that polymer shrinkage is not predictable at a particular location of a tool, particularly where the tool varies in cross section along its length. Furthermore, the uneven cure exotherm of thermosetting resins such as epoxies can cause a large tool to warp or loose strength. In the past, no satisfactory way was known to prevent or compensate for uneven exotherm in very large cast tool bodies.
While it is relatively easy to compensate for polymer shrink in very small tools, it is a very serious problem when one wants to make large tools such as punch and die sets for stamping sheet steel automotive body panels. For example, epoxy resins based on aliphatic amine-cured diglycidyl ethers of bisphenol-A exhibit shrinkage of about 0.9% when unfilled, and from about 0.4 to 0.8% when filled with 20 weight percent of various fillers (see Plastics Materials and Processes, Schwartz and Goodman, Van Nostrand Reinhold Company, 1982, page 365). A tool two meters in length molded from such epoxy would therefore be expected to shrink as much as 8 to 18 mm. Accordingly, large molds could not be cast to size directly from patterns.
The shrink problem of large cast tools has been addressed in a number of ways. For example, U.S. Pat. No. 2,836,530 to Rees discloses a method of tool making in which the surface of the mold for casting the tool is coated with a thin spacing layer of an easily removable material. A hardenable resin is cast into the mold and solidified. During solidification, the cast body shrinks. Rees compensates for this shrinkage by withdrawing the hardened body from the mold and removing the spacing layer from the molding surface. The body is then replaced in the mold and a layer of surfacing material the thickness of the removed spacing layer is added between the body and the mold. When this surfacing layer hardens, it compensates for the shrinkage of the precast large body.
This method creates a number of problems. Because the tool surface is bonded to the body of the tool only after it has already set, there is always a danger of delamination. Moreover, this two-step molding process is cumbersome and time consuming.
U.S. Pat. No. 3,239,590 to Trimble handles the shrinkage problem in a different way. A thin layer of wear-resistant plastic is applied over the surface of the tool mold and is allowed to harden. A reinforcing layer of thermosetting plastic and reinforcing binder is then built up behind the facing layer. Finally, a thin parting film is applied over the built-up and cured layers and a thermosetting plastic resin is poured over the film to fill the tool mold. This resin shrinks as it cures.
After the poured resin has set, it and the parting film are removed from the mold. The film is stripped away and the shrunk cast backing is replaced in the mold over a layer of uncured thermosetting resin. This resin compensates for shrink in the backing and adhesively bonds the facing layer and backing together.
An obvious disadvantage of this method is that it is time consuming. Another problem is that the adhesive bond between the backing and facing would be the weakest point of the tooling and would be subject to fracture under the high pressure impact conditions of metal stamping.
U.S. Pat. No. 4,423,094 to Dearlove et al and assigned to General Motors Corporation, relates to a tough, durable, epoxy novolac material for use in making metal stamping dies. While this material exhibits good characteristics for tooling, it requires a two-step cure. It must first be hardened at room temperature and then post cured at an elevated temperature of about 150.degree. C. It would be preferable to make dies that need no elevated temperature cure. It would also be preferable to mold tools from resins less expensive than epoxy novolacs.
Accordingly, a new method of making large, cast-to-size epoxy tools for metal stamping and polymer molding applications has been developed. More particularly, a method of making such tools directly from a full scale model of the article to be stamped or molded has been invented.